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Several reports in recent years have shown health benefits of dairy products, despite their fat content. The latest research shows an unusually striking effect on weight as children go through their teens.

Pediatricians say too much weight is now the most common medical condition of childhood. The problem has doubled over the past two decades, and about 15 percent are now considered overweight or obese.

Two servings a dayWhile the overall cause is too much food and too little exercise, many studies are attempting to tease apart the precise changes in habits that are driving this health hazard. Several were reported Thursday at a meeting in San Francisco of the American Heart Association.

Lynn Moore, an epidemiologist at Boston University School of Medicine, found that just two servings of dairy food a day are linked to a substantial reduction in adolescent fatness.

Childhood dairy intake has been failing for the last 20 years, in part as youngsters’ preferences have switched from milk to soft drinks. During this time, soda consumption has risen by 300 percent.

Another factor, though, has been fat phobia. Youngsters “consume less and less as they get older,” Moore said. “Adolescent girls in particular are concerned about eating dairy because they think it will make them fat.”

However, her research, based on the Framingham Children’s Study, found just the opposite is true. The analysis was financed largely by the National Health, Lung and Blood Institute with additional funding from the National Dairy Council.

Several studies — including Moore’s — have shown that children and adults who consume adequate amounts of dairy foods have lower blood pressure. Some researchers have put adults on diets with increased dairy and found, to their surprise, that they also seem to lose weight.

Not an excuse to overdo itIn the latest study, the researchers did frequent dietary surveys on 106 families with children and followed them an average of 12 years. They judged body fat by measuring the skin thickness on four parts of their bodies.

They found that those who consumed less than two servings a day averaged about an extra inch of fat in a fold of skin, a surprisingly large amount. The children’s average skin fold thickness was 75 millimeters, while those who ate little dairy were 25 millimeters greater.

Dr. Stephen Daniels, associate chairman of the heart center at Cincinnati Children’s Hospital, noted that the benefit was seen with a relatively modest amount of dairy food, and overdoing it could mean large amounts of extra fat calories.

“You shouldn’t take home from this that you need to eat as much dairy as you can, but it should be part of an overall healthy diet,” he said.

He also noted that no study has yet shown that adding milk to youngsters’ diet actually helps them control weight. He said those who get regular dairy foods may weight less because they eat more home-cooked meals or have breakfast each morning.

Among other findings of Moore’s study:

Youngsters who ate moderate amounts of fat — between 30 percent and 35 percent of total calories — weighed less than those who ate either more or less.

Increased consumption of fruits and vegetables was also associated with lower weight.

Contrary to one popular theory, the glycemic index of children’s diet — the amount of fast-burning carbohydrates — had no bearing on their eventual weight gain.

Just how dairy food might moderate weight gain is a mystery. Moore speculated that calcium or some other nutrient in milk might help influence the way the body stores energy in fat cells. Or perhaps dairy foods simply make children feel less hungry.

Moore noted some parents who cannot eat dairy foods also withhold milk from their children, although lactose intolerance is usually not a problem among the young.

Weight gain in early infancyAnother study at the conference, presented by Dr. Nicolas Stettler of Children’s Hospital of Philadelphia, suggests the important of early infancy weight gain. He followed 1,850 pairs of full-term siblings to help sort out the effects of household and genetic factors.

Six percent of those who gained between eight and 10 pounds during their first four months of life were obese by the time they reached age six, compared with 3 percent of those who put on between six and eight pounds during this critical early period.

He said the work suggests that breast-feeding, which leads to less quick weight gain than formula feeding, may help prevent later obesity.